1. Field of the Invention
The present invention relates to a printing apparatus, particularly to a printing apparatus for forming an image using a signal modulated for each picture element, such as a laser printer, and more particularly to a printing apparatus capable of changing the density of picture elements.
2. Description of the Prior Art
A laser printer is hereinafter described as an example of a printing apparatus. A laser printer forms an image by developing a large number of picture elements arranged in a matrix. In the laser printer, a laser beam is modulated to be applied or shut off dependent on the desired information and serves to form a latent image on a photoconductor drum. The image is caused to be visible through development of toner, whereby it is subsequently transferred onto plain paper and then fixed. Since a laser printer is capable of modulating laser beams at high speed, printing of letters or graphics can be effected with high speed and high quality (high density). Accordingly, laser printers are widely utilized as output apparatus of various data processing systems or picture forming systems using computers.
Images outputted from a host computer or the like have different densities and in order to print images adequately upon receipt of those outputs, it is necessary to control the density of the picture elements in a printer in a variable manner according to those outputs. In addition, in order to change a size of letters to be printed by using a character generator having an identical arrangement of picture elements, the density of picture elements in the printer needs to be variable.
In order to meet such requirements, laser printers having a variable density of picture elements have been proposed (for example, Japanese Patent Laying-Open Gazette No. 198076/1984).
However, such a conventional printer is still inconvenient to use.
More specifically, if a density of picture elements is to be changed from a new page for images successively processed by a host computer or the like, the host computer outputs not only data of those images but also control data for a request for changing the density of picture elements to a laser printer. In such a case, the laser printer is not permitted to change the density of picture elements during an exposure process and accordingly the host computer determines whether exposure is being effected or not in the laser printer, and if exposure is being effected, the host computer waits for an end of the exposure to issue the request for changing the density of picture elements. Thus, signal processing for changing the density of picture elements in the host computer is complicated and the processing speed is caused to slow down.
In addition, in a laser printer disclosed in the above stated Japanese Patent Laying-Open Gazette No. 198076/1984, a rotating speed of a polygon mirror is changed to change a density of picture elements. On that occasion, the number of revolutions of the polygon mirror driven at high speed by a motor is changed and it takes time for the rotation of the polygon mirror to become stable. If an exposure process is started during the unstable period, a disturbance occurs in an image and accordingly the image can not be printed in a good condition. In order to avoid such phenomena, timing of an image density change request signal transmitted from a host computer or the like to the laser printer should coincide with timing calculated for causing the rotation of the polygon mirror to be stable before the start of the exposure process in the laser printer. However, it is difficult to output the image density change request signal with such adequate calculated timing and it is further necessary to allow supplementary time for the output timing. As a result, loss in time is caused and an operating efficiency of the printer is lowered.
Furthermore, an initial density value of picture elements set at turn-on of power supply is fixed in a conventional printer and the user can not change it arbitrarily. Consequently, if a host computer or the like not adapted for the initial density of picture elements is connected, the user must change the density of picture elements after initialization each time the printer is used. In addition, in a laser printer of a type in which change in the density of picture elements involves change in the number of revolutions of a polygon mirror, it takes time to cause the rotation of the polygon mirror to be stable after the number of revolutions of the polygon mirror rotating at high speed has been changed and this causes loss in time.